Robust half-metallic character and large oxygen magnetism in a perovskite cuprate

The new perovskite cuprate material Sr8CaRe3Cu4O24, which behaves ferrimagnetically and shows an unusually high Curie temperature (T(C) approximately 440 K), is found from density-functional theory calculation to display several surprising properties after hole doping or chemical substitution: (1) Half metal (HM) is realized by replacing Re with W or Mo while T(C) remains high; (2) hole-doped Sr8CaRe3Cu4O24 is also HM with high T(C). Moreover, we find that the O atoms will carry a large magnetic moment after hole doping, which is in sharp contrast to the generally accepted concept that magnetism in solids requires partially filled shells of d or f electrons in cations. The material Sr8CaRe3Cu4O24 is therefore expected to provide a very useful platform for material design and development.     

掺杂Sr8 CaRe3 Cu4 O24:一种可能具有半金属性和大的氧磁矩的材料

用密度泛函理论方法仔细研究了新型钙钛矿铜氧材料Sr8CaRe3Cu4O24的电子结构和磁性性质，发现掺杂将使得这种材料表现出非常奇特的性质：（1）用W或Mo替代Re，将使得材料变成半金属，同时它的磁转变温度仍然很高；（2）对Sr8CaRe3Cu4O24进行空穴掺杂，也将使它变为具有高磁转变温度的半金属材料。更为奇特的是，空穴掺杂将导致O原子上有很大的磁矩。因此，Sr8CaRe3Cu4O24很有可能是一种有广阔应用前景的磁性材料。     

Extremely large magnetoresistance in boron-doped silicon

Boron-doped Si-SiO2-Al structures are fabricated to study extremely large magnetoresistance (MR) effects. Current-voltage characteristics show a nonlinear behavior, dominated by an autocatalytic process of impact ionization. At low temperatures, the magnetic field postpones the onset of impact ionization to higher electric fields. This results in a symmetric positive MR of over 10,000% at 400 kA/m. Applying a magnetic field leads to an increase of the acceptor level compared to the valence band as deduced by admittance spectroscopy. A macroscopic transport model is introduced to describe how the MR is controlled by voltage, electrode spacing, and oxide thickness.     

Intergranular giant magnetoresistance in a spontaneously phase separated perovskite oxide

We present small-angle neutron scattering data proving that, on the insulating side of the metal-insulator transition, the doped perovskite cobaltite La(1-x)Sr(x)CoO(3) phase separates into ferromagnetic metallic clusters embedded in a nonferromagnetic matrix. This induces a hysteretic magnetoresistance, with temperature and field dependence characteristic of intergranular giant magnetoresistance (GMR). We argue that this system is a natural analog to the artificial structures fabricated by depositing nanoscale ferromagnetic particles in a metallic or insulating matrix; i.e., this material displays a GMR effect without the deliberate introduction of chemical interfaces.     

Giant magnetoresistance in Ce-doped manganite systems

The fascinating properties like giant magnetoresistance (GMR) effect, metal-insulator transition, charge ordering phenomenon etc. have made the divalent ion doped RMnO₃ (R = rare-earth elements) an attractive system for investigation. Resistivity of these compounds shows a peak near the ferromagnetic transition temperature (T _c ). The application of magnetic field inhibits the spin-disorder scattering and the resistivity decreases drastically. Keeping electrondoped superconductor Nd_2?x Ce _x CuO₄ in mind we have doped RMnO₃ (R = La, Pr, Nd) with tetravalent Ce ion. These compounds are very susceptible to the annealing treatment and belong to the orthorhombic perovskite phase. They show a very high value of resistivity at the peak and under the magnetic field the GMR effect is observed. For La_0.7Ce_0.3MnO₃ and Pr_0.7Ce_0.3MnO₃ the magnetoresistance ratio reaches about 54% and 82.5% respectively at 7.7 T. With the increase of the temperature the magnetic state changes from ferromagnetic to paramagnetic regime. This magnetic transition is not very sharp and the resistivity peak appears at a temperature higher than T _c .     

A local formalism for the study of very large bonded systems

An effective Green function is introduced which satisfies an equation whose solution for a very large system is tractable. This function yields local densities of states and energy-dependent charge densities. Using a tight binding Hamiltonian and basis orbitals localized in the bonds, bulk and intrinsic unrelaxed vacancy states in Ge are studied. Results obtained are comparable to pseudo-potential calculations requiring self-consistency.     

Theory of negative magnetoresistance in three-dimensional systems

Theory of negative magnetoresistance in two-dimensional systems due to delocalization of electrons by magnetic field by Hikami, Larkin, and Nagoaka is extended to the case of three-dimension. The increase in conductivity by magnetic field is independent of the direction of the current relative to that of the magnetic field, and is proportional to $\text{H}{}^2\text{τ}{}_{\mathrm{?}}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$, τ_? being energy relaxation time of electrons, when the magnetic field H is small. If τ_? is large enough at low temperature and H is not too small, it is independent of the parameter characterizing the system, and is of the form $\text{0.918?H mho cm}{}^{\mathrm{?1}}$ (H in kOe).     

Analysis of colossal magnetoresistance effect in perovskite oxide heterostructures

A systemic study of magnetoresistance (MR) in manganite perovskite oxide p-n junction is performed with experiment and theoretical calculation. The spin-dependent tunneling current is calculated with a model of double-band barrier and MR with reverse bias is explained as a result of competition between tunneling currents with different spins. The reduction of recombination rate at the interface of heterojunction with magnetic field is proposed to explain positive MR at forward bias. Furthermore, negative MR is predicted to be observed in oxide heterostructure without electron filling in t2g↓ band of manganite at the interface region with both forward and reverse bias.     

First-principles study of the thermal properties of half-metallic ferromagnetic systems

The origin of the half-metallicity is different in diluted magnetic semiconductors and Heusler alloys. I briefly review our earlier work on (GaMn)As and (GaMn)N focusing on the relation between the half-metallicity and the strength of the interatomic exchange interactions. This relation is governed by the properties of the valence-band holes. In Heusler alloys the factors determining the thermal behavior are distinct. Here the relation between half-metallicity and the longitudinal fluctuations of atomic moments is considered. The temperature dependence of the Ni magnetization in NiMnSb is studied.     

Room-temperature large photoinduced magnetoresistance in semi-insulating gallium arsenide-based device

It is still a great challenge for semiconductor based-devices to obtain a large magnetoresistance(MR) effect under a low magnetic field at room temperature. In this paper, the photoinduced MR effects under different intensities of illumination at room temperature are investigated in a semi-insulating gallium arsenide(SI-Ga As)-based Ag/SI–Ga As/Ag device. The device is subjected to the irradiation of light which is supplied by light-emitting diode(LED) lamp beads with a wavelength in a range of about 395 nm–405 nm and the working power of each LED lamp bead is about 33 mW. The photoinduced MR shows no saturation under magnetic fields(B) up to 1 T and the MR sensitivity S(S = MR/B) at low magnetic field(B = 0.001 T) can reach 15 T~(-1). It is found that the recombination of photoinduced electron and hole results in a positive photoinduced MR effect. This work implies that a high photoinduced S under a low magnetic field may be obtained in a non-magnetic semiconductor device with a very low intrinsic carrier concentration.     

Finite-state neural networks. A step toward the simulation of very large systems

Neural networks composed of neurons withQ _N states and synapses withQ states are studied analytically and numerically. Analytically it is shown that these finite-state networks are much more efficient at information storage than networks with continuous synapses. In order to take the utmost advantage of networks with finite-state elements, a multineuron and multisynapse coding scheme is introduced which allows the simulation of networks having 1.0×10⁹ couplings at a speed of 7.1×10⁹ coupling evaluations per second on asingle processor of the Cray-YMP. A local learning algorithm is also introduced which allows for the efficient training of large networks with finite-state elements.     

On the possibility of suppressing a counterpropagating wave in optical systems

The problem of suppressing a counterpropagating wave when pulse propagates in linear or nonlinear medium or system due to interference quenching at the input boundary with an additional pulse incident on the output boundary has been analyzed. A way of unambiguous determination of the profile of the additional pulse is described for the case of optical media without spatial dispersion.     

The effect of colossal magnetoresistance in SmCu3Mn4O12 perovskite-like oxide

Journal of Experimental and Theoretical Physics - The magnetoresistance effect in the SmCu3Mn4O12 compound with a perovskite-like structure is investigated for the first time. It is found that an...     

Appearance of inverse giant magnetoresistance in noncollinear magnetic systems

We show for a simple d-band TB Hamiltonian that noncollinear magnetic configurations can contribute to large inverse giant magnetoresistance (IGMR) ratios. We make a systematic study as a function of band filling, magnetic moment and canting angle for some simple model examples and use the outcome of this study to interpret the experimentally observed IGMR ratios on LaMn₂Ge₂.     

The wetting angle of very small and large drops

On the basis of Born-Green-Yvon integral equations for the density distribution functions, an approximate integral equation is established for the profile of the surface of the drop. Numerical solutions and analytical solutions for limiting cases are obtained for this profile. Equations relating the angle at the leading edge and in its vicinity to parameters characterizing the interaction forces between the molecules of the liquid and between those of the liquid and solid are derived for large and for very small drops on a horizontal solid surface. One concludes that there is a rapid spatial variation of shape near the leading edge, that for large drops the measured macroscopic wetting angle is reached at a distance of about 20 to 40 Å from the leading edge, and that for very small drops the wetting angle is weakly size dependent. A condition for drop stability is established, which if not satisfied, the liquid will spread over the surface of the solid.     

Two-stage deep learning-based hybrid precoder design for very large scale massive MIMO systems

Wireless networking is approaching a new era, which necessitates new frequency ranges and novel strategies. With recent circuit growth, communications over the Terahertz (THz) band is proving to be a viable option because of the tremendous bandwidth and low cost. On the other hand, THz signals suffer from significant direction loss, necessitating the use of precoding. In this paper, Deep Learning (DL) based precoding techniques for upcoming 6G networks were examined, along with their complexities. Based on the signal-to-noise ratio (SNR) and spectral efficiency (SE), the proposed DL-based precoding scheme is compared to traditional model-based precoding schemes. The proposed DL-based precoding technique is ideal for 6G networks, according to simulation results. Furthermore, the proposed DL-based precoding technique has lower computational complexity, making it suitable for parallel processing and high-speed data transmission.